(15) And from the day on which you bring the sheaf of elevation offering—the day after the sabbath—you shall count off seven weeks. They must be complete: (16) you must count until the day after the seventh week—fifty days; then you shall bring an offering of new grain to the Lord.

From then on [i.e., beginning with the second day of Passover] we are to count forty-nine days[of the omer], which are seven weeks comparable to the [seven] “days” of the world, and then to sanctify the “eighth day” [i.e., the Festival of Weeks] just as the eighth day of Tabernacles [is holy]; and [the forty-nine days] counted between them are in the “intermediate days” of the festival, in the interval separating the first day and “eighth day” of the festival, this being the day of the Giving of the Torah.

It was said, That Rabbi Akiva had twelve thousand pairs of disciples, from Gabbat to Antifaras; and they all died at the same time because they did not treat each other with respect. The world remained desolate until Rabbi Akiva came to our Masters in the South and taught the Torah to them. and it was they who revived the Torah at that time. These were Rabbi Meir, Rabbi Judah, Rabbi Yossi, Rabbi Shimon and Rabbi Eleazar ben Shammua; And it was they who revived the Torah at that time. A Tanna taught: All of them died between Passover and Pentecost. Rabbi Hama bar Abba or, it might be said, Rabbi Chiyya bar Abin said: All of them died a cruel death. What was it? — Rabbi Nahman replied: Croup.

(1) It is customary not to get married between Pesach and Shavuot, until Lag BaOmer (the 33rd day), because during that time, the students of Rabbi Akiva died. However, to do "erusin" and "kiddushin" (engagement and betrothal) is OK. And even for "nisuin" (marriage), if someone did so, we do not punish him. Rema: however, from Lag Ba'Omer onwards, all this is permitted (Abudraham, Beit Yosef & Minhagim).

(2) It is customary not to cut one's hair until Lag BaOmer, since it is said that that is when they stopped dying.

"They said about Rabbi Akiva that he had twelve thousand pairs of students and they all died in one period of time, because they did not treat each other with respect. And the world was desolate of Torah until Rabbi Akiva came to our Rabbis in the South and taught them - Rabbi Meir, Rabbi Yehudah, Rabbi Yosei, Rabbi Shimon, and Rabbi Elazar ben Shamua. And these are the very ones who upheld the time." And it is mentioned here that these [earlier] students all died from Pesach until Shavuot, but it is a tradition in the hands of the Geonim that the dying stopped on the day of Lag BaOmer - so as a result of that, we are accustomed not to fast on it. And we are also accustomed, as a result of that, not to marry a woman from Pesach [only] until that time.

One should add to the joy of the day of the thirty third of the Omer in honor of Rabbi Shimon bar Yochai, may his merit protect us - as it is the day of his memory (the day he died). And it is known that it was his will that [people] rejoice on this day - as is known from the story of our teacher, the Rabbi Avraham HaLevi (Amigo), may his memory be blessed for life in the world to come, and from other stories that we have heard. And we know them from the mouths of holy rabbis. And there is one who is accustomed to engaging in study on the night of the thirty third of the Omer with ten [men] to teach the praises of Rabbi Shimon bar Yochat that are scattered in the Zohar and the Idra Zuta; and that is a nice custom.

The Gemara concludes: And the halakha is that these days were nullified, and the halakha is that they were not nullified. The Gemara asks: This is difficult, as one halakha contradicts the other halakha. The Gemara answers: It is not difficult. Here, it is referring to Hanukkah and Purim. These Festival days were never nullified, and Hanukkah is listed among the Festivals of Megillat Ta’anit. There, the halakha is referring to the rest of the days listed in Megillat Ta’anit, all of which were nullified.